Molybdenum centers probes

Electronic absorption spectroscopy charge transfer transitions, 19 71 d-d transitions, 19 70, 71 flavocytochrome b, 36 269-271 intraligand transitions, 19 71-80 of organometallics, 19 69-80 Electronic coupling, between donor and acceptor wave functions, 41 278 Electronic nuclear double resonance spectroscopy, molybdenum center probes, 40 13... 

M-F coupling constants in, 4 245 Mo, molybdenum center probes, 40 16 multinuclear, tetracyano complexes containing oxo or nitrido ligands, 40 303-304... 

Finally, the recent development of suitable expression systems for recombinant molybdenum enzymes makes possible the use of site-directed mutagenesis to incisively probe the roles of specific active site residues in catalysis. Although these systems have been somewhat slow in development, presumably due to the intricacies of assembling and inserting the molybdenum center into the overexpressed apoenzyme, at least two such systems have now been developed and more can be expected in the immediate future. Future work can be expected to critically evaluate the involvement of specific amino acid residues in each step of a given catalytic sequence. 

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

Photoelectron spectroscopy (PES) has been shown to provide a convenient probe of metal ion effective nuclear charge and the nature of the metal-ligand bond via the energy of valence-electron photoionizations (16, 20, 22, 284, 285, 312, 332-334). Recently, PES spectroscopy has been employed in the study of oxo-molybdenum compounds of the type (L-A5)MoE(X,Y) [E = O, S, NO X, Y = halide, alkoxide, or thiolate] in order to evaluate the synergy between the axial (E) and equatorial (X,Y) donors in affecting the ionization energy of the HOMO localized on the Mo center (16, 284, 334). These studies have conclusively shown that equatorial dithiolene coordination electronically bulfers the Mo center in (L-A pMoEttdt) (Fig. 13) from the severe electronic perturbations associated with the enormous variation in the Ji-donor/acceptor properties... 

The MoFe-protein is an a2 2 tetramer (with the subunits coded by the nifD and nifK genes, respectively), with a total molecular weight of 240,000. The two subunits are of similar size for example, the isolated a and /3 subunits of A. vinelandii MoFe-protein have 491 and 522 amino acids, respectively (46). In general, the amino acid sequences of MoFe-proteins are less well conserved than are Fe-protein sequences, so that the MoFe-protein sequences from A. vinelandii and C. pasteuria-num are only 36% identical (47). Associated with the MoFe-protein tetramer are approximately 2 molybdenum atoms, 30 iron atoms, and 30 sulfur atoms that are organized into two types of metal centers the FeMo-cofactor and the P-cluster pair. The structures and properties of these centers have been extensively probed by a wide variety of techniques. 

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